1. Field of the Invention
This invention relates to a process for producing chlorine dioxide by directly feeding to a chlorine dioxide generator an electrochemically produced aqueous solution of an alkali or alkaline earth metal chlorate substantially free of chromium values and the processing of waste gas streams produced during the production of chlorine dioxide to enable their use as feed solutions for an electrochemical cell for the production of an alkali or alkaline earth metal chlorate.
2. Description of the Prior Art
An aqueous solution of sodium chlorate and sodium chloride is conventionally produced by the electrolysis of aqueous sodium chloride in diaphragmless electrolytic cells. The extent of electrolysis is controlled to produce an effluent from the cell in which the sodium chlorate and sodium chloride have the desired ratio, usually in the range of about 1:1 to about 20:1 and preferably in the range of about 2:1 to about 15:1. The aqueous solution may be further processed to crystallize out the sodium chlorate for sale in crystal form for a variety of purposes. For example, in the wood pulp processing industry, chlorine dioxide which is used in the bleaching of chemical cellulosic pulps is prepared by reduction of aqueous sodium chlorate in the presence of a strong mineral acid, usually, sulfuric acid.
In the electrolysis of sodium chloride to form sodium chlorate, it is conventional to add chromates in the hexavalent state, usually in the form of sodium bichromate dihydrate, Na.sub.2 Cr.sub.2 O.sub.7.2H.sub.2 O, to the electrolyte in the cell to improve significantly the current efficiency of the cell in the conversion of sodium chloride to sodium chlorate. The sodium chlorate containing cell effluent, also known as "cell liquor", therefore, generally contains significant amounts of chromate ions.
It is desirable to remove chromate ions from the cell effluent before employment of the same in chlorine dioxide generation especially in those processes in which methanol is used as a reducing agent in the reduction of chlorate to chlorine dioxide and it is desirable to recover chromate ion for reuse in the electrolytic cells. In addition, chromate ions are a toxic pollutant, so that environmental considerations require removal of the chromate ions where discharge of an effluent stream containing such ions may be effected. A number of prior proposals have been made for the removal of chromate ions from sodium chromate containing cell liquor.
Prior art methods for treating electroplating wastewater or other aqueous liquids containing hexavalent chromium ions as a contaminant are of interest. Electroplating wastewaters often bear heavy amounts of metal contaminates such as copper, cadmium, nickel, and chromium. While these heavy metals readily form hydroxides or sulfides, with the notable exception of chromium, the removal of chromium generally requires an additional treatment step to reduce the chromium ions from the hexavalent to the trivalent state prior to precipitation.
Among the chemicals used in the treatment of wastewater for reducing hexavalent chromium ions to the trivalent state, it is known to use ferrous sulfate, sodium bisulfite, sulfur dioxide, and sodium sulfide. While these chemicals work well as reactants for reducing the hexavalent chromium ions to the trivalent state, the quantity of sludge produced by each of these reactants can vary drastically. Since it is no longer sufficient merely to produce clean water, the volume of sludge produced for disposal is nearly as important as the effluent quality. It is known from U.S. Pat. No. 4,705,639 that in the disclosed sodium sulfide/ferrous sulfate treatment of wastewater for chromium ion reduction, the rate of chromium ion reduction depends upon the pH of the wastewater. In this process the electroplating wastewater is adjusted to a pH of about 8 to 10, treated with sodium sulfide and, thereafter, treated with ferrous sulfate or ferrous chloride to reduce the hexavalent chromium ions to the trivalent state. A process for the removal of hexavalent chromium ions from wastewater is also disclosed in U.S. Pat. No. 4,260,491 in which an aqueous composition containing hexavalent chromium ions and a chelating agent for trivalent chromium ions is treated at about pH 5 with both (1) a known reducing agent for converting hexavalent chromium ions to trivalent chromium ions and (2) with ferric or aluminum chloride or sulfate. This treatment is effective in producing a precipitate of chromic hydroxide upon raising the pH to about 7.5 to about 10.
In the processes discussed above for the removal of dichromates from electroplating baths and other metal treatment solutions, chloride ion is, typically, absent from such metal treatment solutions and, accordingly, very low levels of chromium ions can be obtained in the treated plating baths by reduction of the chromium ions contained therein from the hexavalent state to the trivalent state followed by the precipitation of the trivalent chromium ions as hydrated chromic oxide. The problem of achieving low levels of chromium ions in liquid solutions containing chloride ions is made more difficult by the fact that hydrated chromic oxide has a solubility product higher by a factor of 10.sup.5 when in the presence of chloride-containing solutions.
An aqueous solution of sodium chlorate, produced by the electrolysis of sodium chloride and prepared as a feed for a chlorine dioxide generator of the Rapson R-2 or R3/SVP process type, generally, has a chloride content of about 200 grams per liter. This level of salt content has not proved disadvantageous in these chlorine dioxide generators. However, the R8 and SVP-methanol chlorine dioxide generators benefit in production capability by the use of sodium chlorate solutions having a low chloride ion content. In addition, the chlorine dioxide aqueous solutions produced in SVP-methanol type generators contain less chlorine in the aqueous chlorine dioxide aqueous solution when low chloride content solutions of sodium chlorate are used as feed solutions. In order to obtain the increased production capabilities of the SVP-methanol type chlorine dioxide generators, the costly time, energy, and space consuming steps of crystallizing and redissolving sodium chlorate to make a low chloride content feed solution for the R8 and SVP-methanol type chlorine dioxide generators have been required.
More recently, it has been found possible to obtain low chloride ion content aqueous solutions of sodium chlorate by the use of cascading electrochemical cells, connected in series, having specially designed anodes. These cells which are often supplied for on-site chlorate solution production in the wood pulp mill by the assignee of the instant application are particularly adapted for providing an aqueous solution of sodium chlorate to R8 and SVP-methanol type chlorine dioxide generators which will allow these generators to operate at high production efficiency while producing aqueous chlorine dioxide solutions containing low chlorine content. Accordingly, it is to such low chlorine content aqueous solutions of sodium chlorate to which the process of this invention is particularly adapted.
Low levels of chromium ions are required in electrolytic cell liquors in cells for the production of alkali metal chlorates, particularly, sodium chlorate, in order to increase the current efficiency of the cells, for instance, from about 70% current efficiency to about 95% current efficiency. In addition, the presence of low levels of chromium ion in electrolytic chlorate cells inhibits the formation of explosive mixtures of hydrogen and oxygen. It has been estimated that for every ton of sodium chlorate solution prepared in electrolytic chlorate cells, about 2-10 kilograms of sodium dichromate are present as a contaminate where no effort is made to remove the sodium dichromate.
Although the sodium dichromate has no effect on chlorine dioxide production by reduction of sodium chlorate utilizing the Rapson R-2 process, in the more modern R8 and SVP-methanol chlorine dioxide processes, the presence of chromate ions reduces process efficiency by preventing the smooth operation of the chlorine dioxide generating process. That is, the presence of chromate ions inhibits the crystal growth of the sodium sesquisulfate by-product of the process and thus, makes more difficult its removal. In addition, the environmental impact of the discharge of sodium dichromate in the pulp mill effluent is a serious environmental concern in view of the fact that a 100 thousand tons per annum sodium chlorate plant producing 10 percent of its product as cell liquor would discharge the equivalent of some 20-100 tons per annum of sodium dichromate.
There are a number of methods in the prior art for the removal of alkali and alkaline earth metal chromates from cell liquors. Prior art methods include reduction of the chromate to mixed chromous and chromic salts with the precipitation of insoluble hydroxides in processes in which reduction is effected by water soluble sulfides, hydrazine, hydroxylamine, sulfites, ion exchange techniques, such as those processes disclosed in U.S. Pat. No. Re. 30,081, and Canadian Patent No. 1,035,874 and U.S. Pat. No. 3,980,751, precipitation of chromates, such as those processes disclosed in U.S. Pat. No. 4,086,150; Canadian Patent Nos. 1,124,676 and 1,133,641, and electrochemical means. Many of these processes present problems which make their use either uneconomic or otherwise undesirable. Some of these processes are discussed in copending U.S. application Ser. No. 07/923,378, filed Jul. 31, 1992, incorporated herein by reference.
Chlorine dioxide is an oxidation agent and an important bleaching agent in the pulp industry where it is the most common bleaching agent used in the final stages of pulp bleaching. Recently, there has been an increased use of chlorine dioxide instead of other bleaching agents especially hypochlorite and chlorine. It is known that the use of chlorine in bleaching pulp leads to the production of dioxins which are released to the environment in the disposed wastes. Hypochlorite solutions, when used in bleaching pulp, lead to the formation of chloroform which cannot be tolerated in the paper industry at any concentration. Accordingly, there is a decreased use of chlorine and hypochlorite as bleaching agents in the wood pulp industry. Chlorine dioxide is usually prepared by the reduction of an aqueous solution of sodium chlorate. Modern methods of preparation utilize methanol as a reducing agent in accordance with the following formula: ##STR1##
The gases chlorine and carbon dioxide are produced as by-products in the Rapson R-8 process. Passage of these gases through a sodium hydroxide scrubber produces an aqueous liquor stream of sodium chloride, sodium hypochlorite and sodium carbonate. It is essential from an economical as well as an environmental point of view that this liquor be utilized. A process for conversion of these waste gases comprising chlorine and carbon dioxide to (1) carbon dioxide free of chlorine and (2) an aqueous solution of sodium chlorate is disclosed in copending U.S. application Ser. No. 07/924,546, filed Jul. 31, 1992, incorporated herein by reference.
Alkali metal chlorate, and in particular sodium chlorate has been produced by the electrolysis of aqueous solutions of alkali metal chlorides, such as sodium chloride, in electrolytic cells equipped with or without membranes or diaphragms. Typically, electrolytic cells make chlorates within the cell by reacting chlorine produced at the anode with alkali metal hydroxide produced at the cathode. Representative electrolytic cells of this type are shown in U.S. Pat. No. 3,732,153 and in U.S. Pat. Nos. 3,819,503; 3,791,947; 3,819,504; 3,864,237, and 3,915,817. Various other arrangements of both electrochemical and combinations of electrochemical and chemical methods for manufacturing chlorates have also been proposed, such as the use of a two compartment permselective membrane equipped electrolytic cell operating in conjunction with a diaphragmless-type electrolytic chlorate cell. This method is disclosed in U.S. Pat. No. 3,897,320 to E. H. Cook. However, to obtain improved current efficiencies and significant reductions in electrical power requirements in the production of inorganic chlorate, U.S. Pat. No. 3,464,901 provides for the electrochemical preparation of chlorine and caustic soda in a diaphragm type chloralkali cell. The caustic soda containing unreacted alkali metal chloride and alkali metal chlorate is then removed from the cell and mixed and chemically reacted with chlorine from the anolyte of the cell. The chemical reaction is carried out at a pH of 6 to 8 to convert the alkali metal hypochlorite to chlorate. However, in order to maintain the conditions most favorable for converting hypochlorite to chlorate, additional caustic and/or acid over and above that supplied by the cell has to be added to the reaction mixture. In the case of Japanese Pat. No. 792,025 dilute chlorine is reacted with less than 20 percent caustic soda to produce a concentrated sodium hypochlorite solution with sufficient caustic remaining in it to produce a pH of 8 to 10. The solution is subsequently diluted from about 13 to 15 percent sodium hypochlorite to 6 to 8 percent sodium hypochlorite with a recycled stream of alkali metal chloride and chlorate. The diluted stream is then acidified with hydrochloric acid to a pH of about 6.0 and finally fed to an electrolysis cell.
In U.S. Pat. No. 4,175,038 to Sakowski, a process is disclosed for reducing the available chlorine content of aqueous waste streams, especially calcium hypochlorite waste streams. In the process of this reference, the available chlorine content is reduced by chlorinating the impure stream at a temperature in the range of about 80.degree. to 100.degree. C. at a pH in the range of about 5.5 to about 8.5. During this reaction, the available chlorine is reacted to form the corresponding chlorate.
In U.S. Pat. No. 4,159,929 to Grotheer, a process is disclosed for producing alkali metal chlorates by the reaction of an aqueous solution of an alkali metal chloride, alkali metal chlorate and an alkali metal hypochlorite with an alkali metal hydroxide. Chlorine is added to the reaction mixture in an amount sufficient to maintain the pH of the reaction mixture at about 5-7.5 in order to promote the conversion of alkali metal hypochlorite to alkali metal chlorate. Subsequently, the reaction product is led to an electrolysis cell for the production of an alkali metal chlorate. Instead of feeding brine to the electrolytic sodium chlorate cells, the feed solution is made by reacting a sodium hydroxide solution with chlorine at neutral pH to make a weak sodium chlorate solution which is then electrolyzed in electrochemical cells to a strong sodium chlorate solution. Gaseous chlorine is added to the caustic in an in-line mixer at 70.degree.-80.degree. C. in an amount such that the pH of the mixture is controlled at 5.0-7.5. The resulting hypochlorite solution is then held in an aging tank to allow the hypochlorite to convert to chlorate.
The chlorine and chemical feeds to the process of Grotheer are relatively pure and no provision is made to deal with situations where carbonates may be present, such as tail gases from a methanol type chlorine dioxide generator. The system does not show any way to purify or handle effluent gases which may emanate from the chlorine/caustic reaction, nor does it show any way to deal with foam which would accompany such gases. Also, the in-line mixer and aging tank are not vented and any gases emanating from the reaction, such as CO.sub.2, could create unsafe pressures. Venting these vessels would release chlorine and hypochlorite to the atmosphere, but employing a scrubber would return such gases as CO.sub.2 to the system as carbonate or bicarbonate which would build up to the saturation point and shut the process down.
Also, the process of Grotheer does not produce a chlorate solution low enough in hypochlorite concentration to be purified by conventional means, such as ion exchange. This means that dilute hypochlorite solutions, such as would be discharged from a chlorine tail gas scrubber, would not be suitably treated to be, for example, saturated with NaCl and purified for recycle to electrolytic sodium chlorate cells.
In summary, the process of Grotheer is not able to handle chlorine tail gases or hypochlorite solutions which contain CO.sub.2 or carbonates, nor is the process able to produce a chlorate solution which can be recycled in a closed loop system unless the component chemicals are very pure.
In U.S. Pat. No. 4,216,195 to Jaszka, the production of chlorine dioxide having a low chlorine content is disclosed. A separation technique is utilized in which a gaseous product stream from a chlorine dioxide generator is scrubbed with an aqueous salt mixture containing an approximately stoichiometric quantity of sodium hydroxide. The scrubbing media is a controlled solution of sodium chlorate, sodium chloride and sodium hydroxide which is free of carbonate. The process is not applicable for processing of hypochlorite effluent streams. The sodium hydroxide reacts preferentially with the chlorine in the gas stream, yielding chlorine dioxide of high purity and converting the chlorine to sodium chlorate and sodium chloride which may then be recirculated to a chlorine dioxide generator.
Various processes are disclosed in the prior art for the destruction of hypochlorite, for instance, by reacting the hypochlorite with an acid to produce chlorine, U.S. Pat. No. 4,404,179; the reaction of chlorine with hydrazine in U.S. Pat. No. 3,823,225; or the reaction of an alkali metal hypochlorite with urea, U.S. Pat. No. 4,508,697.
In U.S. Pat. No. 4,620,969 to Wilkinson, a process is disclosed for the production of chlorine by the electrolysis of an aqueous solution of sodium chloride. In part of this process, a gaseous stream containing chlorine and carbon dioxide are passed into a first reaction vessel and thence into a second reaction vessel and aqueous sodium hydroxide solution is charged to the first reaction vessel and aqueous sodium hydroxide is separately charged to the second reaction vessel. An aqueous solution containing sodium hypochlorite is removed from the first reaction vessel and an aqueous solution containing an alkali metal carbonate is removed from the second reaction vessel.
In U.S. Pat. No. 4,129,484 to Larsson, a process is disclosed for the utilization of residual solutions obtained from a chlorine dioxide reactor in which sodium chlorate is reduced to chlorine dioxide in the presence of an acid. The residual solutions are converted to chlorate by leading the residual solutions to an electrolytic cell having at the anode region of the cell an acid enriched fraction of the residual solution.